Study On Dean Vortices And Its Heat Transfer Enhancement Characteristics In Helical Tubes

According to previous researches on heat transfer enhancement in the tube side of shell and tube heat exchangers, issues such as high pressure drop and comprehensive performance of heat transfer enhancement are the major challenges in the application of heat transfer enhancement in tube side. Based on the heat transfer theory, one typical kind of secondary flow─Dean Vortices is introduced, whose flow characteristics and heat transfer enhancement mechanism are studied in this paper through numerical simulation and experiment research.Numerical simulation results of the flow in helical and straight tubes showes that the average radial velocity of the flow in helical tubes increases with Reynolds number, viscosity and curvature ratio but decreases with the torsion, owing to the formation of Dean Vortices even if under low Reynolds number conditions. However, the average radial velocity of the flow in straight tube approximates zero. The average wall shear stress and pressure drop in helical and straight tubes both increase with the Reynolds number and viscosity. However, either the average shear stress or the pressure drop is much higher than that of the flow in helical tubes. Moreover, average wall shear stress and pressure drop in helical tubes increase with the curvature ratio of helical tubes. In addition, as torsion of helical tubes increasing, the average wall shear stress increases by a small margin, and the pressure drop decreases by a small range. And the torsion plays another role was to change the symmetry of Dean Vortices.Heat transfer experiments results showe that Nusselt number and pressure drop in helical tubes both increase with curvature ratio, Reynolds number and viscosity, and they are much higher than that of the flow in straight tubes under the same curvature ratio, Reynolds number and viscosity. Moreover, the influence of the torsion on Nusselt number is nonsignificant, but it could cause the pressure decreasing to some extent. The values of evaluation factor of heat transfer enhancement in helical tubesηare larger than 1 at any given curvature ratio, torsion, Reynolds numbers and viscosity. However, the value ofηdecreases with the increase of curvature ratio(ηhas higher values when a/Rc≤0.16), and increases within a certain Reynolds number range but decreases afterward(the values ofηare relatively high when the Reynolds number ranges between 1520 and 5000), meanwhile having unclear varing trend with the increase of torsion. It just grows with the the increase of the fluid viscosity(the value could go up to 1.5), which suggests that the helical tubes have a better comprehensive performance of heat transfer enhancemennt than straight tube, especially under the fluid with high viscosity.The mechanism of heat transfer enhancement by Dean vortices in helical tubes mainly attributes to the convection effect and shearing action of Dean vortices. In addition, relationships between Nusselt number and Dean number, Pressure drop and Dean number, evaluation factorηand Dean number are obtained on the basis of experiment results. And the correlation equations are obtained by multiple regression method.Flow characteristic of Dean Vortices and its heat transfer enhancement mechanism are further studied in this paper, which could improve the heat transfer performance in tube side and is of great significance to the design and engineering application of shell and tube heat transfer exchangers.